DESCRIPTION (Applicant's Abstract): Mutations in the genes encoding presenilins 1 and 2 (PS-1 and PS-2) are a leading cause of familial, early-onset Alzheimer's disease (AD). Studies manipulating presenilin expression and introducing mutant forms into transgenic mice and cultured cells have demonstrated an important role for PS-1 in early brain development and suggested potential pathogenic mechanisms for the mutations, but have technical limitations which constrain their utility. The proposed research will extend these efforts by evaluation of the normal and pathogenic roles of PS-1 in the maturing and aging brain, using novel mouse models and cultured neurons derived from them. We use gene targeting to create mouse experimental models that emulate faithfully the genetics of familial Alzheimer's disease (mutant "knock-in") or underexpress PS-1 into adulthood ("hypomorph"). Five mouse lines will be studied: (1) PS-1 and APP wild type; (2) PS-1P264L knock-in; (3) APPswe knock-in; (4) PS-1/APP double knock-in; (5) PS-1 hypomorph. Specific Aim 1 will define functional roles of PS-1 in mouse brain maturation and aging and test whether an FAD-linked PS-1 mutation or partial loss of PS-1 function cause Alzheimer-type degenerative neuropathology. Comparative histological analyses will evaluate effects of knock-in of the FAD-linked PS-1P264L mutation or PS-1 hypomorphism on neuronal death rates and the size, topology and regional architecture of the maturing and aging mouse brain, and will address cellular and biochemical bases for PS-1-related neuropathologies. Specific Aim 2 will test the hypothesis that an AD-linked mutant PS-1, when expressed at normal levels, endangers brain neurons in vitro and in vivo to degeneration. Neuronal vulnerability to atrophy, apoptosis and necrosis will be evaluated as a function of PS-1 genotype for cultured primary neurons of different maturational states and for the injured adult brain. Specific Aim 3 will test the hypothesis that an AD-linked mutant PS-1 increases production in brain of the amyloid Abeta1-42 peptide by enhancing recruitment of a fragment of the beta-amyloid precursor protein. The rate-limiting step in Abeta1-42 formation will be determined by molecular and pharmacologic analyses of cultured primary neurons, and predictions made by the "recruitment hypothesis" will be evaluated critically in cultured neurons and the mouse brain. These studies will advance our understanding of normal and pathogenic functions of PS-1 in the brain, and so provide an important foundation for developing therapeutic strategies aimed at slowing the progressive deterioration of Alzheimer's disease.